Recently the identification of mutations in several genes involved in the onset of obesity in rodents have been identified. Of particular interest are mutations discovered in the peptide hormone, leptin, which is a component of a novel signal transduction pathway that regulates body weight (Zhang et al. 1994, Nature 372:425-432; Chen et al. 1996, Cell 84:491-495). Leptin was initially discovered by the positional cloning of the obesity gene, ob, in mice. Two different ob alleles have been identified: one mutation causes the premature termination of the leptin peptide resulting in a truncated protein, and the other mutation changes the transcriptional activity of the obesity (ob) gene, resulting in a reduced amount of circulating leptin.
There is a correlation between a decrease in the levels of biologically active leptin and the overt obese phenotype observed in oblob mice. Recombinant leptin has been shown to induce weight loss in the ob/ob mouse but not in the diabetic phenotype db/db mouse (Campfield et al. 1995, Science 269: 546-549; Halaas et al. 1995, Science 269: 543-546; Pellymounter et al. 1995, Science 269:540-543; Rentsch et al. 1995, Biochem. Biophys. Res. Comm. 214:131-136; and Weigle et al. 1995, J. Clin. Invest. 96:2065-2070).
Although the synthesis of leptin occurs in the adipocyte, its ability to decrease food intake and increase metabolic rate appears to be mediated centrally by the hypothalamus. Injection of recombinant leptin into the third ventricle of the brain elicits a similar response as peripheral administration of leptin. Furthermore, the recent cloning of the human receptor for the leptin, the ob-receptor (OB-R), reveals that it is transcribed in the hypothalamus (Tartaglia et al. 1995, Cell 83:1263-1271; Stephens et al. 1995, Nature 377: 530-532). In addition, a mutation that results in premature termination of the long-form of the mouse OB-R, which is preferentially expressed in the hypothalamus, appears to be responsible for the obese phenotype of the db/db mouse (Lee et al. 1996, Nature 379:632-635; Chua et al. 1996, Science 271:994-996; and Chen et al. 1996, Cell 94:491-495).
The OB-R from wild type (lean) rats and from rats having the fatty mutation (both heterozygous and homozygous fa ) have been isolated and sequenced. (patent application Ser. Nos. 60/146,928 and 60/013,969, now 08/803,346, pending filed Feb. 22, 1996 and Mar. 22, 1996, which are hereby incorporated by reference.) The gene encoding the OB-R of the fatty rat bears a missense mutation such that its expression product differs from the wild-type rat OB-R amino acid sequence shown in SEQ ID NO: 15 by the substitution Gln.sup.269 .fwdarw.Pro, as shown in SEQ ID NO: 20.
Various isoforms of the OB-Rs have also been identified. These isoforms are due to alternative splicing. For example, in the mouse the a form has 5 amino acids following the Lysine at 889; the b form has 273 amino acids after Lysine 889; the c form has 3 amino acids after Lysine 889; and the d form contains 11 amino acids after Lysine 889.
It would be desirable to be able to further experiment with various isoforms in order to better understand obesity, and to be able to clone and produce novel oh receptor isoforms to use in assays for the identification of ligands which may be useful in understanding obesity and for its prevention and treatment.